Boost limit question (PSi limit)
 

Can someone explain to me the relationship between the boost limit of the ECU and the limit set by the wastegate acutuator? i.e. My car (96 WRX) has a boost limit of 12PSI (rounded for ease of typing) I know that the ECU will allow the car to boost to 12PSI and the wastegate actuator releases boost at 12PSI BUT if I upgrade the chip on the ECU to allow 16PSI doesn't the wastegate actuator still release at 12PSI - I cannot see any physical link between the ECU and the actuator so how do they "sing from the same hymn sheet"???

Thanks

the boost is controlled by the boost control solenoid BCS this controlled by your ecu and will try and maintain boost by bleeding from the boost control solenoid to achieve 16psi.

This is where you are going wrong. There is a physical link between the ECU and wastgate actuator.

Unless your car has been modified to eliminate it, the boost control solenoid valve is plumbed into the pipework between the compressor bleed air outlet and the wastegate actuator. The solenoid is controlled by the ECU.

While the actual science of boost control is quite complex, simplifying it as much as possible and picking up your analogy, if you recalibrate the ECU to run a 16psi boost target, what you will find in practice is that it will wait until the turbo is generating close to 16psi before it allows pressure into the wastegate actuator.

OK, I understand the link between the ECU and wastegate actuator via the BCS BUT, I can physically change the actuator by shortening the actuator rod, which I did to move the boost from an indicated 10 PSI back to 12PSI (recomended as the actuator can go "soft" with age) how does that link in to the BCS?

As above this is a complex science and tbh your best bet would be to click search and go from there.

Again trying to keep it simple, the connection between ECU and turbo via the boost solenoid is a bit like a hose with water flowing through it. The turbo is the source of the water, with the solenoid being the tap. If you are holding the hose, and too much water is flowing through it, you'll shut the tap. If you want more water, you'll open it up a little more.

In this case, the ECU has a specific boost target in mind, it will adjust the duty cycle of the solenoid up or down in order to achieve that target. If the actual bost is lower than the target, the duty cycle will be increased. If the target has been overshot, the ECU will lower the DC to bring boost down again.

In this way it can automatically adjust, within certain limits, for an actuator that is a little "hard" or "soft", as well as for changing atmospheric conditions.

What had obviously happened before is that your actuator had gone soft to the point the ECU could no longer compensate via the solenoid.